The architecture of a conventional hermetically sealed coax type feed-through RF connector is diagrammatically illustrated in cross-section in FIG. 1 as comprising a longitudinal pin or center signal conductor 10 of conductive material, such as KOVAR (KOVAR is a federally registered trademark of Carpenter Technology Corporation and will hereinafter be denoted as KOVAR (Reg. Tdmk)), that lies along the axis 12 of the RF connector. Pin 10, as well as the remaining components of the RF connector, are cylindrically symmetrical about axis 12. A first portion 11 of pin 10 is shown as being surrounded and hermetically sealed by a dielectric material, such as a generally cylindrical glass member 20, from one end of which projects an interior distal end 13 of the pin 10. A second portion 14 of the pin passes through a bore 31 in a Teflon bushing 30 and terminates at an exterior distal end 15. The glass member 20 adjoins and is hermetically sealed against a generally cylindrical KOVAR (Reg. Tdmk) ferrule 40 having a generally ‘L’ shaped cross-section, that facilitates welding of the ferrule to a compatible metallic ring portion of an adjoining outer shell 50. Ferrule 40 includes an annular depression 41, which is typically referred to in the industry as a stress-relief cut, or thermal isolation groove. The outer diameter of a base portion 42 of the ferrule 40 is sized to fit within and be captured by a generally cylindrical slot or depression 51 within a first end portion 52 of outer shell 50.
The outer shell 50 is typically made of duplex material, in particular two dissimilar metals, and includes a first or main body portion 53 of a first metal such as aluminum to facilitate welding the shell to the next high layer of the assembly, such as to an adjacent (aluminum) support housing 60, and a second body portion 54 which adjoins the base portion 42 of the ferrule and is made of a material that is metallurgically compatible with the material of the ferrule—in this case KOVAR (Reg. Tdmk), for example, so as to facilitate laser welding of the shell to the ferrule and thereby providing the intended hermetic seal therebetween. The first (aluminum) and second (KOVAR (Reg. Tdmk)) body portions of the shell 50 are typically joined together by explosion welding.
Region 70 represents a portion of a laser weld used to metallurgically join the base portion 42 of the KOVAR (Reg. Tdmk), ferrule with the second body portion 54 of the outer shell 50. It is to be understood that the laser weld 70 forms an annular weld joint completely around the adjoining portions of the KOVAR (Reg. Tdmk), ferrule 40 and the KOVAR (Reg. Tdmk), portion of shell 50. A depiction of the material of the weld 70 has been omitted from the lower portion of FIG. 1 in order to show the pre-weld shapes of the components.
An electrically conductive contact spring 80 is captured along the outer reduced diameter portion 45 of the KOVAR (Reg. Tdmk) ferrule 40 and serves as a portion of the conductive path for the cylindrical ground plane that surrounds the center pin 10. In addition, an electrically conductive flexible gasket 90, such as a rubber gasket impregnated with metallic (e.g., silver) particles, is retained within an annular depression 56 within the shell 50 so as to maintain intimate contact with the shell and the glass-sealed feed-through, and like spring 80, serves as a portion of the conductive path for the ground plane. The electrically conductive gasket is necessary since the only positive contact between the shell 50 and the glass-sealed feed-through is the laser weld 70, which must be located away from the glass to prevent heat damage of the glass during welding. It may also be noted that those portions of the RF connector where signal travels are coated with a highly conductive metal such as gold.
Now although the purpose of the RF connector architecture of FIG. 1 is to provide a low expansion feed that passes through a high electrical performance material (glass), into a high expansion package, it suffers from its large size, a mismatch of the electrical path lengths along the relatively long and undulating ground path and the relatively straight center conductor, which the ground plane surrounds, formation of the laser weld is labor intensive and the laser weld region itself is subject to corrosion. The large size is due to the fact that the diameter of the feed-through must be increased to accommodate the thermal isolation groove to prevent damage to the glass seal due to heat during welding. The increase in ground path length is due to the combination of materials employed in order to ensure a continuous ground path through the entirety of the RF connector.
This signal and ground plane length mismatch may be understood by reference to FIG. 2, which shows a bold straight arrow 200, which represents the signal path that is established along the plated outer surface of center pin 10, and by way of the bold undulating arrow 210 which traverses the interior plated regions of shell 50 and ferrule 40, as well as the electrically conductive gasket 90 and the contact spring 80 (the purpose of which is to ensure that the ground path is continuous). In order for the coax feed-through to electrically function properly, the ground path should be as straight and short as possible so as to match the distance traversed by the signal along the center pin. It may be noted that if the electrically conductive gasket 90 were omitted from the RF connector architecture of FIGS. 1 and 2, the resulting structure, shown in FIG. 3, would cause the ground path length to increase very substantially, producing an “out of phase” RF signal which could severely degrade or null the RF transmission.
The susceptibility of the laser weld to corrosion is due to the fact that the there is no hermetic seal between the center pin and the Teflon bushing, so that ambient moisture is able to leak along the exterior joints of the RF connector and reach the laser weld 70 between the iron-containing material of the base portion 42 of the KOVAR (Reg. Tdmk) ferrule 40 and the iron-containing material of the second body portion 54 of the shell. Moreover, formation of the laser weld is labor intensive, since the region where the weld is to be made must be masked prior to plating the surfaces of the ferrule and the outer shell. The weld masking material must then be removed in order to perform the welding operation on the bare (KOVAR (Reg. Tdmk)) metal of the outer shell and the ferrule.